U.S. patent application number 14/172745 was filed with the patent office on 2016-08-04 for moving platform roll sensor system.
This patent application is currently assigned to TELEDYNE SCIENTIFIC & IMAGING, LLC. The applicant listed for this patent is TELEDYNE SCIENTIFIC & IMAGING, LLC. Invention is credited to Brian GREGORY, Dong-Feng GU, Milind MAHAJAN, Donald TABER, Bruce K. WINKER.
Application Number | 20160223365 14/172745 |
Document ID | / |
Family ID | 56554065 |
Filed Date | 2016-08-04 |
United States Patent
Application |
20160223365 |
Kind Code |
A1 |
MAHAJAN; Milind ; et
al. |
August 4, 2016 |
MOVING PLATFORM ROLL SENSOR SYSTEM
Abstract
A moving platform roll sensor system comprises an ellipsometric
detector capable of detecting a polarized beam within the
detector's line-of-sight, and measuring the beam's polarization
state, such that the polarization state indicates the rotational
orientation of the moving platform with respect to a predefined
coordinate system. The ellipsometric detector comprises a venetian
blind component through which the polarized beam passes, arranged
such that the intensity of the exiting beam varies with its
incident angle with respect to the moving platform, a polarizing
beamsplitter which splits the exiting beam into components having
orthogonal circular polarizations, the relative intensities of
which vary with the relative polarization vector of the beam, and
first and second detectors which receive the first and second
orthogonal circular components and generate respective outputs that
vary with the intensities of their received components. The
beamsplitter preferably comprises a quarter wave plate and a
polarization grating.
Inventors: |
MAHAJAN; Milind; (Thousand
Oaks, CA) ; WINKER; Bruce K.; (Ventura, CA) ;
TABER; Donald; (Newbury Park, CA) ; GREGORY;
Brian; (Newbury Park, CA) ; GU; Dong-Feng;
(Thousand Oaks, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TELEDYNE SCIENTIFIC & IMAGING, LLC |
Thousand Oaks |
CA |
US |
|
|
Assignee: |
TELEDYNE SCIENTIFIC & IMAGING,
LLC
Thousand Oaks
CA
|
Family ID: |
56554065 |
Appl. No.: |
14/172745 |
Filed: |
February 4, 2014 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01D 5/3473 20130101;
G01D 5/345 20130101 |
International
Class: |
G01D 5/34 20060101
G01D005/34; G01D 5/347 20060101 G01D005/347 |
Goverment Interests
GOVERNMENT RIGHTS
[0001] This invention was made with Government support under DARPA
contract HR0011-09-C-0016. The Government has certain rights in
this invention.
Claims
1. A moving platform roll sensor system, comprising: a moving
platform, said platform comprising an ellipsometric detector
capable of detecting a polarized beam of electromagnetic radiation
when said ellipsometric detector is within the line-of-sight of
said polarized beam, and of measuring said polarized beam's
polarization state such that said polarization state indicates the
rotational orientation of said moving platform with respect to a
predefined coordinate system; said ellipsometric detector
comprising: a polarizing beamsplitter which receives said detected
beam and splits said beam into first and second components having
orthogonal circular polarizations, the relative intensities of
which vary with the relative polarization vector of said detected
beam; and first and second detectors arranged to receive said first
and second components having orthogonal circular polarizations,
respectively, and to generate respective outputs that vary with the
intensities of their received components.
2. The system of claim 1, wherein said beam is a linearly polarized
laser beam.
3. The system of claim 1, wherein said polarizing beamsplitter
comprises a quarter wave plate and a polarization grating.
4. The system of claim 3, wherein said polarization grating
comprises a holographically-treated liquid crystalline
material.
5. The system of claim 1, further comprising a venetian blind
component through which said detected polarized beam passes, said
component arranged such that the intensity of the beam after it
passes through the component varies with the incident angle of said
detected beam with respect to said moving platform.
6. The system of claim 5, wherein said ellipsometric detector
further comprises a protective window through which said detected
polarized beam passes prior to reaching said venetian blind
component.
7. The system of claim 6, further comprising a coating applied to
said protective window to filter out wavelengths outside the
spectral range of said polarized beam's electromagnetic
radiation.
8. The system of claim 5, wherein the polar angle-dependent
transmission characteristic of said venetian blind component is
arranged such that orientational ambiguity in the ellipsometer is
resolved.
9. The system of claim 5, wherein said venetian blind component
comprises first and second intensity gratings spatially separated
from each other.
10. The system of claim 9, wherein said first and second intensity
gratings are located on opposite sides of a substrate.
11. The system of claim 9, wherein said first and second intensity
gratings have respective duty cycles and share a common period.
12. The system of claim 11, wherein said second intensity grating
is offset with respect to said first intensity grating.
13. The system of claim 9, wherein said at least one of first and
second intensity gratings comprises a reflective element to block
light and an absorptive element to attenuate internally reflected
light.
14. The system of claim 5, wherein said venetian blind component
and said polarizing beamsplitter are formed as a monolithic
structure, said structure comprising: a single substrate having an
input side and an exit side; first and second intensity gratings
fabricated on said input and exit sides, respectively, to form said
venetian blind component; a quarter wave plate fabricated or placed
on said input side or said exit side of said substrate; a
polarization grating fabricated or placed on said input side or
said exit side of said substrate, said quarter wave plate and said
polarization grating arranged such that an incoming beam impinges
on said quarter wave plate before impinging on said polarization
grating.
15. The system of claim 14, wherein said quarter wave plate is
fabricated or placed on said input side of said substrate and said
polarization grating is fabricated or placed on said exit side of
said substrate.
16. The system of claim 1, wherein said ellipsometric detector
further comprises a lens through which said detected polarized beam
passes and which focuses said first and second components onto said
first and second detectors.
17. The system of claim 16, wherein said lens has an f-number of
F/3.0 or less.
18. The system of claim 1, wherein the polarization grating of said
polarizing beamsplitter is arranged such that >90% of the light
exiting said beamsplitter goes to +1 order or -1 order, with
<10% of said light being zero order.
19. The system of claim 1, wherein said moving platform further
comprises a retroreflector arranged to reflect said polarized beam,
said system further comprising a detector, an array of detectors,
and/or a camera arranged to receive said reflected beam.
20. The system of claim 1, wherein the output D1 of said first
detector is proportional to cos.sup.2.theta. and the output D2 of
said second detector is proportional to sin.sup.2.theta., where
.theta. is the rotational orientation of said moving platform with
respect to said predefined coordinate system, with .theta. given
by: .theta. = cos - 1 D 1 D 1 + D 2 . ##EQU00002##
21. The system of claim 1, wherein said first and second detectors
are sized to detect incoming polarized beams having an angle of
incidence of +/-.alpha.degrees, where .alpha. is from 2-30
degrees.
22. The system of claim 1, wherein said moving platform has an
associated center axis and said ellipsometric detector is arranged
such that it is weight- and rotation-balanced around said center
axis.
23. The system of claim 1, further comprising a means of dithering
the polarization state of said polarized beam such that the
polarization of said polarized beam is occasionally or periodically
rotated.
24. The system of claim 1, wherein said polarized beam is generated
by an transmitter and a free space link is established between said
transmitter and said ellipsometric detector when said ellipsometric
detector is within the line-of-sight of said polarized beam.
25. The system of claim 24, further comprising a phase-locked-loop
(PLL) circuit coupled to said ellipsometric detector and arranged
to track said rotational orientation and thereby mitigate the
degradation in the accuracy of said rotational orientation
determination that might otherwise occur when said link is
disrupted.
26. The system of claim 1, wherein said polarized beam is generated
by a transmitter, said transmitter further arranged to encode
guidance commands into said beam by pulsing said beam, said moving
platform arranged to detect and decode said pulses and thereby
detect said guidance commands.
27. The system of claim 26, wherein said moving platform is
arranged to vary its spatial orientation in response to said
guidance commands.
28. A moving platform guidance system, comprising: a transmitter
which generates a pulsed beam having a known polarization with
respect to a predefined coordinate system; a moving platform, said
platform comprising an ellipsometric detector capable of detecting
a polarized beam when said ellipsometric detector is within the
line-of-sight of said polarized beam, and of measuring said
polarized beam's polarization state such that said polarization
state indicates the rotational orientation of said moving platform
with respect to a predefined coordinate system; said ellipsometric
detector comprising: a venetian blind component through which said
detected polarized beam passes, said component arranged such that
the intensity of the beam after it passes through the component
varies with the incident angle of said detected beam with respect
to the said moving platform; a polarizing beamsplitter which
receives said detected beam after it passes through said venetian
blind component and splits said beam into first and second
components having orthogonal circular polarizations, the relative
intensities of which vary with the relative polarization vector of
said detected beam, said polarizing beamsplitter including a
polarization grating arranged to create angular separation between
said orthogonal circular polarizations; first and second detectors
arranged to receive said first and second components having
orthogonal circular polarizations, respectively, and to generate
respective outputs that vary with the intensities of their received
components; and a retroreflector arranged to reflect said polarized
beam; and a detector, an array of detectors, and/or a camera
arranged to receive said reflected beam.
Description
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates generally to sensors for determining
the rotational orientation of a platform moving in space.
[0004] 2. Description of the Related Art
[0005] It is often necessary to know the rotational orientation of
a moving body or platform. For example, it may be necessary to know
the orientation of a moving projectile such as a missile in order
to provide the missile with appropriate guidance data.
[0006] Several techniques are used to provide rotational
orientation data of this sort. For example, it may be possible to
determine the orientation of a moving platform by means of a radar
system. However, such systems tend to be large and costly; they
also consume a large amount of power and are easy to detect.
Another approach is to affix accelerometers, gyroscopes,
magnetometers etc. to the platform; however, these devices also
tend to be expensive, bulky and complex.
[0007] It may also be possible to determine the rotational
orientation of a moving platform by imaging it as it moves.
However, this is likely to be difficult if conditions are turbulent
or otherwise less than ideal, and may be impossible if the
projectile is small and rapidly spinning.
SUMMARY OF THE INVENTION
[0008] A moving platform roll sensor system is presented which
addresses several of the problems noted above, providing a robust,
compact, low cost sensor for determining the rotational orientation
of a moving platform.
[0009] The present sensor system is for use on a moving platform.
The system comprises an ellipsometric detector, capable of 1)
detecting a polarized beam of electromagnetic radiation when the
ellipsometric detector is within the line-of-sight of the polarized
beam, and 2) measuring the beam's polarization state, such that the
polarization state indicates the rotational orientation of the
moving platform with respect to a predefined coordinate system.
[0010] The ellipsometric detector comprises: [0011] a venetian
blind component through which the detected polarized beam passes,
arranged such that the intensity of the beam after it passes
through the component varies with the incident angle of the
detected beam with respect to the moving platform; [0012] a
polarizing beamsplitter which receives the detected beam after it
passes through the venetian blind component and splits the beam
into first and second components having orthogonal circular
polarizations, the relative intensities of which vary with the
relative polarization vector of the detected beam; and [0013] first
and second detectors arranged to receive the first and second
components having orthogonal circular polarizations, respectively,
and to generate respective outputs that vary with the intensities
of their received components.
[0014] The beam is a preferably a linearly polarized laser beam,
and the polarizing beamsplitter preferably comprises a quarter wave
plate and a polarization grating. The ellipsometric detector also
preferably comprises a protective window through which the detected
polarized beam passes prior to reaching the venetian blind
component.
[0015] These and other features, aspects, and advantages of the
present invention will become better understood with reference to
the following drawings, description, and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1a is a block diagram of moving platform roll sensor
system in accordance with the present invention.
[0017] FIG. 1b is a diagram illustrating front and side views of a
venetian blind component as might be used with a moving platform
roll sensor system in accordance with the present invention.
[0018] FIG. 1c is a diagram illustrating the components making up
the rotational orientation value .theta..
[0019] FIG. 2 is a block diagram of an ellipsometric detector as
might be used with a moving platform roll sensor system in
accordance with the present invention.
[0020] FIG. 3a depicts plots of detector signal vs. platform
orientation for a moving platform roll sensor system in accordance
with the present invention, with and without the use of a venetian
blind component.
[0021] FIG. 3b is a block diagram of a venetian blind component as
might be used with a moving platform roll sensor system in
accordance with the present invention.
[0022] FIG. 3c is a schematic drawing for one of the possible
arrangement of the films/coatings on a venetian blind that enable
its polarization sensing/discrimination functions.
[0023] FIG. 4 is a block diagram of a moving platform guidance
system which includes a moving platform roll sensor system in
accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The basic principles of a moving platform roll sensor system
per the present invention are illustrated in FIG. 1a. A moving
platform 10, the rotational orientation .theta. of which is
desired, comprises an ellipsometric detector 12 capable of
detecting a polarized beam of electromagnetic radiation 14 when the
ellipsometric detector is within the line-of-sight of the polarized
beam. Optical beam 14 has a known polarization with respect to a
predefined coordinate system. Ellipsometric detector 12 is arranged
to measure the polarization state of the detected beam, which is
used to indicate the azimuthal orientation .theta. of the moving
platform--suitably a projectile--with respect to the predefined
coordinate system.
[0025] Optical beam 14 is preferably a linearly polarized laser
beam. One component of ellipsometric detector 12 is preferably a
venetian blind component 16 through which the detected polarized
beam passes. This venetian blind is a component arranged such that
the intensity of the beam after it passes through the component
varies with the incident angle of the detected beam with respect to
the moving platform. An example is illustrated in the front and
side views shown FIG. 1b, with .theta. being the tangential
component of the angle between the incident beam and the vector
normal to the venetian blind surface. As the platform rotates, the
incident angle changes and transmission through the Venetian blind
varies with this tangential component, which is in the plane
orthogonal to the grating stripes. If the venetian blind material's
refractive index is different from that of the material surrounding
it, the refraction will cause the beam to bend slightly inside the
substrate; .theta..sub.internal is the angle subtended by the
refracted beam to surface normal inside the venetian blind
substrate.
[0026] Ellipsometric detector 12 also includes a polarizing
beamsplitter 18 which receives the detected beam after it passes
through venetian blind component 16, and splits the beam into first
and second components (20, 22) having orthogonal circular
polarizations, the relative intensities of which vary with the
relative polarization vector of the detected beam. Then, first and
second detectors (24, 26) are arranged to receive first and second
components 20 and 22, respectively, and to generate respective
outputs D1 and D2 that vary with the intensities of their received
components, with D1+D2 being proportional to the total intensity of
the detected beam.
[0027] When so arranged, the output D1 of detector 24 is
proportional to cos.sup.2.theta. and the output D2 of photodetector
30 is proportional to sin.sup.2.theta., where .theta. is the
azimuthal orientation of moving platform 10 with respect to the
predefined coordinate system. Thus, azimuthal orientation .theta.
is given by:
.theta. = cos - 1 D 1 D 1 + D 2 . ##EQU00001##
A simplification or an approximation of this expression might also
be used. The components making up .theta. are illustrated in FIG.
1c.
[0028] A preferred embodiment of ellipsometric detector 12 is shown
in FIG. 2. Polarizing beamsplitter 18, located adjacent to venetian
blind component 16, is preferably a conformal design which
comprises a quarter wave plate, which separates the polarized beam
into two orthogonal circular polarizations, and a polarization
grating, with the polarization grating preferably comprising a
holographically-treated liquid crystalline material. This
polarization grating arrangement serves to create angular
separation between orthogonal circular polarizations. One suitable
grating is described in J. Kim, et al, "Wide-angle, non-mechanical
beam steering using this liquid crystal polarization gratings,"
Proc. of SPIE Vol. 7093 709302-1, which describes the fabrication
of a film (.about.1-5 .mu.m thick) on a fused silica substrate. The
film serves as a high efficiency polarization grating with only two
diffraction orders (.+-.1) corresponding to two orthogonal circular
polarizations. The polarization grating of polarizing beamsplitter
18 is preferably arranged such that >90% of the light exiting
the beamsplitter goes to +1 order or -1 order, with <10% of the
light being zero order.
[0029] Ellipsometric detector 12 preferably also includes a
protective window 30 through which detected polarized beam 14
passes prior to reaching venetian blind component 16. Window 30 is
preferably made from a hard material, which is preferably coated or
composed to filter out wavelengths outside the spectral range of
the polarized beam's electromagnetic radiation.
[0030] Ellipsometric detector 12 preferably also includes a lens 32
through which the detected polarized beam passes after passing
through polarizing beamsplitter 18, and which focuses first and
second orthogonal components 20 and 22 onto first and second
detectors 24 and 26, respectively. Lens 32 preferably has an
f-number of F/3.0 or less. Note that a lens could be positioned in
front of beamsplitter 18 rather than behind it, though this is not
preferred. Ellipsometric detector 12 may also include a
retroreflector 34, such as a corner cube, arranged to reflect the
polarized beam 14.
[0031] As noted above, venetian blind component 16 is arranged such
that the intensity of the beam after it passes through the
component varies with the tangential component of the incident
angle of the detected beam with respect to the moving platform.
This angle-dependent transmission characteristic of the venetian
blind component resolves orientational ambiguity (up vs. down,
0.degree. vs. 180.degree.) in the ellipsometer. This is illustrated
in FIG. 3a, which shows plots of detector signal vs. platform
orientation for a moving platform roll sensor system in accordance
with the present invention, with and without the use of a venetian
blind component. With no venetian blind component, the detector
signal's peak intensity is essentially constant over time, making a
determination of up vs. down or 0.degree. vs. 180.degree.
orientation difficult or impossible. However, with venetian blind
component 16 in place, the detector signal's peak intensity varies
over time, with the difference 40 between intensity peaks being
indicative of orientation. Once orientation is determined, a
circuit such as a phase-locked-loop (PLL) (42, shown in FIG. 2) can
be coupled to ellipsometric detector 12 and arranged to track the
rotational orientation and thereby mitigate the degradation in the
accuracy of the rotational orientation determination that might
otherwise occur if the optical link between the polarized beam and
the ellipsometric detector is disrupted.
[0032] The sensitivity of the ellipsometric detector to platform
rotation is at minimum when the rotational orientation is such that
light is primarily directed to one detector. If the orientation of
the moving platform is not varying, and the detected beam is
primarily directed onto just one of detectors 24 and 26, its
orientation may be difficult to track, or the PLL's phase lock may
be lost. This can be addressed by including a means of dithering
polarized beam 14, such that the linear polarization of the
polarized beam is occasionally or periodically rotated. This can
make tracking the orientation of the moving platform easier by, for
example, enabling phase lock on the slowly rotating platform.
[0033] Venetian blind component 16 preferably comprises two
spatially separated intensity gratings affixed to a common
substrate; an exemplary structure is shown in FIG. 3b. Here, a
front intensity grating 50 having a duty cycle DC1 (=0.5 in this
example) and a back intensity grating 52 having a duty cycle DC2
(preferably from 0.1 to 0.4; 0.15 in this example) are mounted on
opposite sides of a substrate 54. Though the front and back
intensity gratings have different duty cycles, they should share a
common period P--though the second grating may be offset with
respect to the first (as shown).
[0034] The substrate 54 is characterized by a thickness t and a
refractive index n; t and n can be chosen to match commonly
available substrates. Transmission through the venetian blind
component distinguishes between incident angles of opposite sign as
shown. Thus, for beam 56, .theta.=+.theta..sub.c, and transmission
T through the component is equal to DC1=0.50; however, for beam 58,
.theta.=-.theta..sub.c, and T=DC1-DC2=0.35, where .theta. is the
angle between the incident beam and the vector normal to the
venetian blind surfaces (incidence angle), and +/-.theta..sub.c are
the angles at which the venetian blind reaches the first maximum
and first minimum transmission values, respectively.
[0035] Assuming that the offset between the front and back
intensity gratings is zero, transmission through the component is
minimal when P/2=t tan(.theta..sub.internal). As noted above, if
the venetian blind material's refractive index is different from
that of the material surrounding it, the refraction will cause the
beam to bend slightly inside the substrate. .theta..sub.internal is
the angle subtended by the refracted beam to surface normal inside
the venetian blind substrate. The angular dependence of
transmission can be biased by introducing an offset between the two
gratings. Overall grating performance is specified by selecting
period P, the offset, and duty cycles DC1, DC2 as needed. The goal
is to obtain adequate contrast between up and down orientation over
the range of incident angles likely to be encountered. The venetian
blind's transmission dependence on incident angle is mathematically
equivalent to the convolution of the two square wave representing
the transmission of the two individual gratings that comprise
it.
[0036] The venetian blind component 16 and polarizing beamsplitter
18 can be fabricated as a monolithic structure. Such a structure
can be formed on a single substrate having an input side and an
exit side. One possible embodiment example is given in FIG. 3c, in
which two intensity gratings are fabricated on opposite sides of
the substrate to form the venetian blind component. To enhance the
polarization discrimination function, it is preferred that at least
one of the intensity gratings (on the left side of substrate 54 in
FIG. 3c) has an absorptive element 51 with a sub-micron thickness
(typically about 0.3-0.8 microns), and a reflective element 53
which preferably has a thickness of about 50-100 nm. The function
of absorptive element 51 is to attenuate the beam component 55
reflected by the reflective element 53 making up the intensity
grating on the right side of substrate 54. Otherwise, an
intra-substrate reflection 57 can leak through and lessen the
intensity difference between beams 56 and 58. The function of the
reflective element is to ensure 100% blockage with a relatively
thin metal layer. A quarter wave plate (66 in FIG. 3c) and a
polarization grating (64 in FIG. 3c) can be placed on either side
of the venetian blind, as long as the quarter wave plate meets the
incoming beam first. However, depending on monolithic fabrication
methods, a preference exists for their placement. If a quarter wave
plate and a polarization grating are made by directly coating
liquid crystal films onto a venetian blind substrate, it is
preferred that the quarter wave plate is placed on the beam input
side (the side with absorptive element 51), and the polarization
grating is placed on the exit side of the venetian blind. When so
arranged, the surface on which the polarization grating is coated
will be smoother, resulting in much less liquid crystal alignment
defects during the polarization grating coating process: the UV
hologram used to create the polarization grating coating is less
perturbed, with lower surface steps and lower elastic train in the
liquid crystal. In contrast to a periodic liquid crystal
orientation change in a polarization grating, the liquid crystal
alignment in a quarter wave plate is uniform, and it has a much
higher tolerance to surface steps. Therefore, the intensity grating
having absorptive element 51 is preferably placed on the input side
of the venetian blind, as shown in FIG. 3c. If a quarter wave plate
and a polarization grating are made separately and then fabricated
or placed onto a venetian blind, their placement can be on either
side of the venetian blind.
[0037] A complete moving platform guidance system may be formed
around the present moving platform roll sensor system, as shown in
FIG. 4. Here, the ellipsometric detector 12 on moving platform 10
may include a retroreflector 60, such as a corner cube, arranged to
reflect the polarized beam 14. A complete system would preferably
further comprise a detector, an array of detectors, and/or a camera
62 arranged to receive the reflected beam 64. The polarized beam is
generated by a transmitter 66, and a free space link is established
between transmitter 66 and ellipsometric detector 12 when the
ellipsometric detector is within the line-of-sight of polarized
beam 14.
[0038] Transmitter 66 may be further arranged to encode guidance
commands into polarized beam 14 by, for example, pulsing the beam.
Components on the moving platform such as ellipsometric detector 12
may then be arranged to detect and decode the pulses and thereby
detect the guidance commands. The moving platform may be arranged
to vary its spatial orientation in response to the guidance
commands.
[0039] Ellipsometric detector 12 is preferably arranged such that
it is weight- and rotation-balanced around the center axis of
moving platform 10. This may be facilitated by using a polarizing
beamsplitter which includes a polarization grating as discussed
above, which allows detectors 24 and 26 to be positioned
side-by-side and rotation balanced.
[0040] The detectors 24 and 26 for the present moving platform roll
sensor system are preferably sized to detect incoming polarized
beams having an angle of incidence of +/-.alpha. degrees, where a
is from 2-30 degrees.
[0041] The embodiments of the invention described herein are
exemplary and numerous modifications, variations and rearrangements
can be readily envisioned to achieve substantially equivalent
results, all of which are intended to be embraced within the spirit
and scope of the invention as defined in the appended claims.
* * * * *